1. Field of the Invention
The present invention relates to an apparatus for producing wideband speech signals from narrowband speech signals and, in particular, relates to an apparatus for producing wideband speech from telephone-band speech.
2. Description of the Related Art
Among prior methods of expanding speech bandwidth, there is the method described in Y. Yoshida, T. Abe, et al. "Recovery of wideband speech from narrowband speech by codebook mapping", Denshi Joho Tsushin Gakkai Shingakuho SP 93-61 (1993) (in Japanese language) and the method described in Y. Cheng, D. O'Shaughnessy, P. Mermelstein, "Statistical recovery of wideband speech from narrowband speech", Proceed. ICSLP 92 (1992), pp. 1577-1580.
According to the method by Yoshida et al. a large number of code words, for instance 512 codes, have been necessary for reliably expanding speech bandwidth, since the method relies on codebook mapping. On the other hand, the method of Cheng et al. had a problem in the quality of the synthesized speech, since white noise, which is not correlated to the original speech, is added.